A problem in use of a coated steel sheet for an exterior building material or the like is the generation of red rust. For example, in a region which does not suffer from salt damage (salt damage-free region), red rust is generated on an exposed base steel portion of a coated steel sheet, such as an edge surface and a bent portion, and the red rust causes a problem of deterioration of appearance. The generation of red rust can be effectively prevented by subjecting a steel sheet to chromate-based chemical conversion treatment or adding a chromic acid-based anti-corrosive pigment into an undercoating film. However, a coated steel sheet which does not cause elution of hexavalent chromium has been recently required from the viewpoint of reduction of environmental loads.
For a technique to prevent the generation of red rust on an exposed base steel portion without using a chromate-based chemical conversion treatment solution or a chromic acid-based anti-corrosive pigment, use of manganese phosphate or manganese phosphite as an anti-corrosive pigment is proposed (see PTL 1). Manganese phosphate and manganese phosphite are eluted onto an exposed base steel portion to form a protective coating film in an early stage. By virtue of this protective coating film, prevention of the generation of red rust on an exposed base steel portion is achieved without using a chromate-based chemical conversion treatment solution or chromic acid-based anti-corrosive pigment.
When a coated steel sheet is used for an exterior building material or the like, scratch resistance is required in some cases. For a technique to enhance the scratch resistance of a coated steel sheet, addition of a silica particle having a particle diameter of 1 to 5 μm into an undercoating film is proposed (see PTL 2). Addition of a silica particle into an undercoating film to increase the surface roughness of the undercoating film broadens the contact area between the undercoating film and a topcoating film, which enhances the adhesion strength of the topcoating film to the undercoating film. As a result, enhancement of the scratch resistance of a coated steel sheet is achieved.